1. Field of the Invention
The present invention relates generally to photovoltaic devices and more particularly to thin film solar cells comprising a thin transparent conducting film of cadmium stannate.
2. Description of the Prior Art
Photovoltaic devices, used extensively in a myriad of applications, have generated considerable academic and commercial interest in recent years. Photovoltaic devices (solar cells) utilize the specific conductivity properties of semiconductors to convert the visible and near visible light energy of the sun into usable electrical energy. This conversion results from the absorption of radiant energy in the semiconductor materials which frees some valence electrons, thereby generating electron-hole pairs. The energy required to generate electron-hole pairs in a semiconductor material is referred to as the band gap energy, which in general is the minimum energy needed to excite an electron from the valence band to the conduction band.
Cadmium telluride (CdTe) has long been recognized as a promising semiconductor material for thin-film solar cells due to its near-optimum band gap of 1.5 eV and high absorption coefficient. CdTe is typically coupled with a second semiconductor material of different conductivity type such as cadmium sulfide (CdS) to produce a high efficiency heterojunction photovoltaic cell. Small-area CdS/CdTe heterojunction solar cells with solar energy to electrical energy conversion efficiencies of more than 16% and commercial-scale modules with efficiencies of about 9% have been produced using various deposition techniques, including close-space sublimation or "CSS" (U.S. Pat. No. 5,304,499, issued Apr. 19, 1994, to Bonnet et al.), spray deposition (e.g., J. F. Jordan, Solar Cells, 23 (1988) pp. 107-113), and electrolytic deposition (e.g., B. M. Basol, Solar Cells, 23 (1988), pp. 69-88).
Thin film solar cells typically comprise an optically transparent substrate through which radiant energy enters the device, the intermediate layers of dissimilar semiconductor materials (e.g., CdS and CdTe), and a conductive film back contact. Generally, when the substrate is not electrically conductive, a thin layer of transparent conductive oxide (TCO) is deposited between the substrate and the first semiconductor layer to function as a front contact current collector. However, conventional TCOs, such as tin oxide, indium oxide, and zinc oxide, have high sheet resistivities (typically about 10 ohms per square), and hence poor conductivity, at thicknesses necessary for good optical transmission. Thus, because of their high sheet resistivities, conventional TCOs are not efficient current collectors in solar cells of any appreciable size (i.e., greater than about one square centimeter), particularly in commercial-scale modules.
One way around the current collection limitation described above is to incorporate a more efficient current collection means, such as a front contact current collector grid, into the TCO layer. These current collector grids generally comprise a network of very low resistivity material that collects electrical current from the transparent conductive layer and channels the current to a central current collector. For example, U.S. Pat. Nos. 4,647,711, 4,595,790 and 4,595,791 to Basol et al. each disclose a photovoltaic device having a metallic conductive grid integrated into the TCO layer to decrease the series resistance of the device. Although supplementing the TCO layer with a metallic grid may theoretically enhance the current collecting capacity of the solar cell, because the grid material is not optically transparent, the presence of the grid can actually reduce the overall conversion efficiency of the photovoltaic device. Other disadvantages and potential problems commonly associated with the use of current collector grids include diffusion of the grid material into the semiconductor layers, short circuiting of the device, and incomplete or uneven deposition of the semiconductor layers due to the geometry of the grid.
U.S. Pat. No. 4,808,242 to Murata et al. discloses a photovoltaic device having a substrate on which a plurality of transparent electrodes for each photoelectric conversion cell are arranged. Each transparent electrode has a coupling conductor and a plurality of collecting electrodes connected to the coupling conductor. Although the Murata et al. device includes a transparent current collecting network, and thus avoids the problems associated with a non-optically transparent system, it is difficult and expensive to produce due to the additional materials and processing steps required to integrate the intricate arrangement of electrodes and coupling conductors.
It is desirable to create a transparent conducting film between the substrate and the first semiconductor layer that has both low electrical sheet resistance and high optical transmission. Low sheet resistance is a primary requirement of any contact on a semiconductor device to reduce the barrier to carrier flow between the semiconductor device and the external electronic circuit. High optical transmission is also very important to increase the amount of electromagnetic radiation that is absorbed by the semiconductor material, thereby optimizing the operation of the photovoltaic device by maximizing the number of photogenerated electrons available for collection. Unfortunately, it is difficult to provide both of these conditions simultaneously, low sheet resistance and high optical transmission, in the transparent conducting layer using conventional methods and TCO materials. As previously stated, conventional TCOs have high inherent resistivity. High sheet resistance causes ohmic losses in the transparent conducting film, which decreases the overall conversion efficiency of the device. To reduce the sheet resistance of these conventional TCO films, and thus potentially improve device performance, the TCO must be deposited as a relatively thick layer. However, the thicker the transparent conducting film, the lower the transmission and thus the less electromagnetic radiation that reaches the semiconductor material, thereby reducing the conversion efficiency of the solar cell.
Another disadvantage associated with conventional TCO layers in thin film solar cell devices is their generally rough surface morphology. For example, one of the most popular TCOs currently in use, tin oxide (SnO.sub.2), when deposited as a thin film by chemical vapor deposition (CVD) typically produces an average surface roughness of between about 100 and 250 .ANG.. Such high surface roughness has several significant disadvantages. First, it is well known that high efficiency solar cells (e.g., thin film CdS/CdTe solar cells) require a very thin semiconductor (CdS) window layer, typically with a thickness of around 600 .ANG.. However, this high SnO.sub.2 surface roughness coupled with a thin CdS layer can significantly affect the uniformity of both the CdS layer and the resulting CdS.sub.1-x Te.sub.x intermixed layer, which will be described in more detail below. If the CdS and CdS.sub.1-x Te.sub.x layers are not uniform or complete, this has the adverse effect of increasing interface defects thus reducing open circuit voltage and fill factor, and can ultimately cause severe degradation. Second, a high surface roughness increases the junction area of the solar cell which causes an increased dark current, and hence a lower open circuit voltage and fill factor. Finally, it is desirable to create a smooth surface on the transparent conducting film so that the thickness of the semiconductor window layer can be minimized. Having a very thin window layer means more absorption of optical photons (particularly energy of short wavelength) in the active region of the semiconductor device, and thus improved photovoltaic conversion efficiency.
Another problem with conventional SnO.sub.2 is that they can be very difficult to pattern, which limits their commercial applications. It is especially important for commercial applications that the transparent conducting film be easy to pattern or etch, particularly for advanced module and display device processing. Transparent conducting films suitable for commercial use must also be easy to produce, inexpensive, durable, stable under standard processing conditions, and chemically compatible with the semiconductor material, specifically the CdS window layer.
A need therefore exists for an improved, high efficiency, thin film photovoltaic device. This improved device should include a transparent conducting film (TCO) which features a variety of desirable optical, electronic and mechanical properties. Specifically, the transparent conducting layer should exhibit high electrical conductivity, high optical transmission, relatively smooth surface morphology, good chemical and environmental stability, be easy and inexpensive to produce, and be easily patternable for module production. Ideally, this high efficiency device should have a front contact with a sheet resistivity as low as 2-5 ohms per square and an optical transmittance greater than 85 percent. Until this invention, no such device existed.